1. Field of the Invention
The present invention relates to a multilayered board and to a method for fabricating the same. More particularly, the invention relates to a multilayered board using a glass-containing material which can be fired at low temperatures, and to a method for fabricating the same.
2. Description of the Related Art
As the level of functionality of electronic components is increased and the size of electronic components is reduced, more and more semiconductor ICs and other surface-mounting devices are required be mounted on circuit boards. In order to meet such requirements, various types of multilayered ceramic boards, which are fabricated by laminating ceramic green sheets with applied electrode paste for forming electrodes, followed by firing, have been developed and widely used.
Recently, multilayered boards (low-temperature-fired multilayered boards) have been developed using glass-containing materials, such as glass-ceramic materials, which can be fired at low temperatures.
When a multilayered board is fabricated, an Ag paste containing Ag powder as a conductive component, for example, is widely used as the electrode paste for forming electrodes.
However, when an Ag paste comprising Ag powder, or Ag powder and an oxide serving as a chemical bonding agent, is applied to a board, in which a large amount of ceramic is included in order to increase the strength, and firing is performed simultaneously, if the glass component content is low in the ceramic layer and in the electrodes, it is difficult to obtain satisfactory electrode bonding strength.
The electrode bonding strength may be increased by adding a glass component to the Ag paste so as to improve its wettability on the board, or by including a large amount of a glass component in the board. However, in such cases, the glass component floats on the surfaces of the electrodes, resulting in a decrease in the solderability of the electrodes or in the flexural strength of the board.
The present invention provides a multilayered board which has a high bonding strength between the electrodes and the glass-containing insulating layers, satisfactory solderability, and a high flexural strength, and provides a method for fabricating the same.
In one aspect of the present invention, a multilayered board includes a laminate including a plurality of glass-containing insulating layers, each glass-containing insulating layer being provided with an electrode on the surface thereof. The glass-containing insulating layer is formed by firing a layer containing 60% by volume or less of a glass component before firing, a portion of the glass component is segregated in the surface region of the glass-containing insulating layer by firing, and the electrode is bonded to the surface of the glass-containing insulating layer by means of the segregated glass component.
In the multilayered board of the present invention, since the glass-containing insulating layer before firing has a glass component content of 60% by volume or less, and the electrode is bonded to the surface of the glass-containing insulating layer by means of the glass component which has been segregated in the surface region by firing, the multilayered board does not have an excessively high glass component content in the glass-containing insulating layer so as to decrease its flexural strength, and it does not have an excessively high glass component content in the electrode so as to decrease its solderability, whereby a high electrode bonding strength can be secured.
xe2x80x9cThe glass component content of 60% by volume or less before firingxe2x80x9d conceptually means that the glass component content in the inorganic component constituting a material for forming the glass-containing insulating layer before firing (green sheet) is 60% by volume or less.
Preferably, in the multilayered board of the present invention, the glass component content is in the range of 60% to 90% by volume within a depth of 1 xcexcm from the surface of the glass-containing insulating layer provided with the electrode.
By segregating the glass component by firing and by increasing the glass component content within a depth of 1 xcexcm from the surface of the glass-containing insulating layer to 60% to 90% by volume, it is possible to improve the electrode bonding strength even if the glass component content in the entire glass-containing insulating layer is not increased. Consequently, it is possible to prevent the glass component content in the entire glass-containing insulating layer from increasing excessively, and a high flexural strength can be maintained, thus ensuring the effectiveness of the present invention.
Preferably, the glass-containing insulating layer is composed of a glass-ceramic material.
In the present invention, although it is possible to use materials in which glass and various other insulating components are mixed as the glass-containing insulating layer, in order to attain various electrical characteristics and mechanical characteristics, a glass-ceramic material containing a glass and a ceramic is preferably used. In such a case, the advantage of the present invention is reliably obtained.
Preferably, the electrode contains Ag as a principal conductive component.
In the present invention, although it is possible to use various materials for the electrode, when a material containing Ag as a principal conductive component is used for the electrode, the electrode can have a low resistance and superior bonding strength, thus ensuring the effectiveness of the present invention.
Preferably, the electrode and the glass-containing insulating layer are simultaneously fired.
Although the electrode and the glass-containing insulating layer may be fired separately, by simultaneously firing, it is possible to improve the bonding strength between the electrode and the glass-containing insulating layer and also the fabrication process can be simplified.
In accordance with the present invention, the initial bonding strength of the electrode to the glass-containing insulating layer can be improved, and the initial bonding strength to the glass-containing insulating layer in a tensile test can be 5 N/mm2 or more on average.
In accordance with the present invention, since the glass component content in the glass-containing insulating layer is not increased to such an extent that the mechanical strength of the glass-containing insulating layer is greatly decreased, the flexural strength which does not present a difficulty in practical use, namely, a flexural strength of 200 MPa or more, can be achieved.
In the multilayered board of the present invention, preferably, the glass component in the glass-containing insulating layer is a crystallized glass.
By using the crystallized glass as the glass component in the glass-containing insulating layer, the mechanical strength can be further improved, thus further ensuring the effectiveness of the present invention.
In another aspect of the present invention, a method for fabricating a multilayered board, in which a plurality of glass-containing insulating layers are laminated, each glass-containing insulating layer being provided with an electrode on the surface thereof, includes the steps of applying an electrode paste for forming electrodes to the surfaces of green sheets containing 60% by volume or less of a glass component; laminating and pressure-bonding the green sheets applied with the electrode paste to form a pressure-bonded laminate; and heat-treating the pressure-bonded laminate so that the green sheets and the electrode paste are simultaneously fired, a portion of the glass component is segregated in the surface regions of the glass-containing insulating layers, and the electrodes are bonded to the surfaces of the glass-containing insulating layers by means of the segregated glass component.
Since the method for fabricating the multilayered board of the present invention includes the steps described above, it is possible to reliably fabricate the multilayered board which has high electrode bonding strength, high flexural strength, and superior solderability of the electrodes.
Additionally, xe2x80x9cthe step of laminating and pressure-bonding the green sheets applied with the electrode paste to form a pressure-bonded laminatexe2x80x9d conceptually includes a case in which the electrodes (electrode paste) of the individual layers are connected to each other through via-holes made in the green sheets.
xe2x80x9cThe green sheets containing 60% by volume or less of a glass componentxe2x80x9d means that the glass component content in the inorganic component constituting the green sheets is 60% by volume or less.
In the method for fabricating the multilayered board of the present invention, one or more of the glass component content in the green sheets, the glass composition, the glass-forming conditions, the pressure-bonding conditions of the green sheets, and the firing conditions may be adjusted so that the glass component content is in the range of 60% to 90% by volume within a depth of 1 xcexcm from the surfaces of the glass-containing insulating layers provided with the electrodes.
That is, in order to avoid a decrease in mechanical strength, the green sheets containing 60% by volume or less of the glass component are used, while by adjusting at least one of the glass component content in the green sheets, the glass composition, the glass-forming conditions, the pressure-bonding conditions of the green sheets, and the firing conditions, the glass component content within a depth of 1 xcexcm from the surfaces of the glass-containing insulating layers to be provided with the electrodes is set in the range of 60% to 90% by volume. Consequently, it is possible to improve the electrode bonding strength by increasing the glass component content only in the surface regions of the glass-containing insulating layers, and it is also possible to maintain a high flexural strength by preventing the glass component content in the entire glass-containing insulating layers from being too high, thus ensuring the effectiveness of the present invention.
Preferably, the green sheets containing 60% by volume or less of the glass component are composed of a glass-ceramic material.
In the present invention, it is possible to use green sheets composed of a material in which glass and various other insulating components are mixed as the green sheets for forming the glass-containing insulating layers. However, in order to attain various electrical characteristics and mechanical characteristics, green sheets composed of a glass-ceramic material containing a glass and a ceramic are preferably used. In such a case, the advantage of the present invention is reliably obtained.
Preferably, an electrode paste containing Ag as a principal conductive component is used as the electrode paste for forming the electrodes.
In the present invention, although it is possible to use various materials for the electrodes, when the electrodes are formed by using the electrode paste containing Ag as a principal conductive component, it is possible to efficiently fabricate a multilayered board in which the electrodes have a high bonding strength to the glass-containing insulating layers and also the electrodes have a low resistance.
Preferably, the electrode paste containing Ag as the principal conductive component does not contain a substantial amount of glass.
By using the electrode paste which substantially does not contain glass as the electrode paste containing Ag as the principal conductive component, it is possible to form electrodes having a low resistance and superior solderability, thus further ensuring the effectiveness of the present invention.
Additionally, since the electrodes are bonded to the glass-containing insulating layers by means of the glass segregated in the surface regions of the glass-containing insulating layers, the electrode bonding strength is secured.
Preferably, the electrode paste containing Ag as the principal conductive component contains at least one material selected from the group consisting of Pb, Bi, Cr, Cu, Mn, Co, and Zn.
By adding at least one material selected from the group consisting of Pb, Bi, Cr, Cu, Mn, Co, and Zn to the electrode paste containing Ag as the principal conductive component, further improvement in the electrode bonding strength is expected.
Preferably, green sheets containing a crystallized glass as the glass component are used.
By using the green sheets containing the crystallized glass, it is possible to fabricate a reliable multilayered board in which the glass-containing insulating layers have a high mechanical strength, and also the entire board has a high mechanical strength, thus further ensuring the effectiveness of the present invention.
In the method for fabricating a multilayered board of the present invention, in the step of heat-treating the pressure-bonded laminate, a constraining layer may be laminated on at least one of the upper surface and the lower surface of the pressure-bonded laminate, the constraining layer being composed of an inorganic material which is not sintered at the firing temperature of the pressure-bonded laminate, and firing is performed in that state, and then the constraining layer is removed, whereby a so-called xe2x80x9cnon-shrinkage processxe2x80x9d may be employed.
By employing the non-shrinkage process when the laminate is fired, it is possible to fabricate a multilayered board which does not shrink in the lateral dimensions, which has a high electrode bonding strength, and which has a superior flexural strength and solderability. By using the multilayered board thus fabricated, it is possible to efficiently produce an electronic component, such as a hybrid IC, in which mounting devices are reliably mounted at desired positions.